1. Field of the Invention
The present invention relates generally to heat transfer equipment, and more particularly to a heat-tracing apparatus.
2. Description of the Related Art
To ensure the continuity of plant operations in industries, such as refining, chemical, and pulp and paper, process and utility pipes and vessels are often heated by convection steam tracers to keep the contents of fluids such as liquids, gases, vapors, suspensions and slurries from freezing, condensing, crystallizing, separating or becoming too viscous to pump.
A standard measurement of steam tracing performance used in industry is conductance. Tracer conductance is designated C.sub.T. The C.sub.T value is derived to reflect the normalized heat transfer coefficient of the heat transfer element (tracer). The coefficient is normalized in terms of a "per unit length," generally reduced to feet. The resulting C.sub.T is commonly given units of BTU/hr-ft-.degree. F.
If Q=U.multidot.A.multidot..DELTA.T, where Q=heat loss (BTU/hr), U=overall heat transfer coefficient (BTU/hr-FT.sup.2 -.degree. F.), .DELTA.T=temperature gradient, and A=surface area, then for purposes of steam tracing, U.multidot.A is defined as the conductance of the tracer (C.sub.T). This eliminates the complex variables involved in calculating U.multidot.A which include variables such as size of the heat transfer element (tracer), size of the process pipe, area of contact between the tracer and the pipe and several other factors. The C.sub.T value is experimentally derived by measuring Q and .DELTA.T on a particular steam tracing system.
There are three methods of steam tracing used when steam is chosen as the tracing method of choice. A first and most prevalent steam tracing method uses bare metallic tracer tubes, and is termed "bare tracing" or "convection tracing." In convection tracing, a small diameter tube or pipe, which contains steam or a hot fluid, is externally secured with wire, banding material or high temperature tape to a process pipe, vessel, or other plant equipment for heating a process fluid therein. The steam tracer may be parallel to or spirally around the process pipe, vessel or other plant equipment to be heated. Thermal insulation is placed over the steam tracer and the process pipe or object being heated.
The steam tracer, typically copper or stainless steel tubing, relies on spot contact between the tracer line and the process pipe together with convection (thus termed "convection tracing") to the surrounding air inside the annular space between the oversized thermal insulation and the process pipe and radiation from the tracer line to the process pipe. The heat transfer conductance range for convection tracers (see Table 1) is dependent upon the method and quality of securement of the tracer to the process pipe.
A 3/8-inch outside diameter (O.D.) convection tracer firmly taped or banded on 1 to 2 foot centers will exhibit a heat transfer conductance of from approximately 0.258 to approximately 0.430 BTU/hr-ft-.degree. F. If the convection tracer and process pipe is taped or banded as described and covered with a suitable metal foil or if the tracer is covered with a rigid metal shield and secured to the pipe, it will exhibit a heat transfer conductance of from approximately 0.430 to approximately 0.603 BTU/hr-ft-.degree. F. It is estimated that approximately 70% to 80% of all steam tracing in North America is bare convection tracing.
A second method of steam tracing is termed "conduction tracing", where the tracer line is thermally bonded to the pipe or equipment with a heat transfer material. The primary goal of conduction tracing is to provide increased heat transfer from the tracer tube to the process pipe or equipment. A conduction steam tracing method is disclosed in assignee's U.S. Pat. No. 3,949,189. It is estimated that approximately 10-20% of steam tracing is conduction tracing.
A third method of steam tracing is called "isolated tracing" and is used to provide less heat than the first and second methods discussed above. This method consists of providing a heat retardant between the steam tracer tube and the pipe or equipment to be heated. Some systems consist of small tubes, generally 1/4-inch to 3/4-inch, that have a factory-applied heat retardant either wrapped or extruded on the tube, generally with a high-temperature, weatherproof jacket to protect the retardant from moisture intrusion. It is estimated that approximately 1-2% of steam tracing is currently isolated tracing. An isolated (retarding) heat tracing method is disclosed in assignee's U.S. Pat No. 5,086,836.
The tracer conductance C.sub.T in conduction tracing methods is dependent in part on the geometric relationship between tracer size and process pipe size. Table 1 shows typical conductance values for convection conduction and isolated tracing systems.
TABLE 1 ______________________________________ Typical Conductance Ranges of Tracers C.sub.T, BTU/hr-ft-.degree. F. Tracer Size Convection Tracer Conduction Tracer Isolated Tracer ______________________________________ 3/8-inch 0.258-0.603 0.611-8.0 0.025-0.500 1/2-inch 0.343-0.80 0.814-8.00 0.27-.700 3/4-inch 0.517-1.2 1.221-8.0 0.35-1.00 ______________________________________
A problem with pipes or tubes carrying steam is that they can present a danger to workers who may be required to work on or around them, due to the high pressures and temperatures inherent with steam. In 1993, there were 19,142 lost time hand injuries due to contact with hot objects or substances (Bureau of Labor Statistics). In 1988, there were 20,941 work-related injuries involving disability from contact with temperature extremes ("Accident Facts" 1993 Edition). In 1986, there were 24,749 work-related injuries involving disability from contact with temperature extremes ("Accident Facts" 1990 Edition).
Workers in manufacturing suffer the greatest number of heat burn injuries, about 40 percent of the total, according to a Bureau of Labor Statistics study of over 1,300 heat burn cases filed as worker's compensation claims. More than 40 percent of the injuries were to the upper extremities. Injuries were most often caused by flame, molten metal, petroleum asphalts, steam or water, and resulted in 2nd or 3rd degree burns in over 85 percent of the cases ("Accident Facts" 1990 Edition).
Thus, work-related burn injuries are a problem in general. Burn injuries from heat tracing is a particular problem, and convection tracing, which is the majority of heat tracing, is the primary source of bums from heat tracing. However, the desirable goal of reducing burn injuries from heat tracing has been somewhat incompatible with the necessity to maintain process fluids at a desired temperature in an economical fashion.